protonmail can exchange pgp-encrypted messages with non-proton addresses, but:
1. it can't fetch their keys from a keyserver. either the external sender has to attach their key, and the proton user has to trust it, or the proton user has to edit the contact card and manually upload the public key.
2. if you encrypt the topic of an email sent to a proton user, they'll be able to view it, but their reply will include the topic unencrypted
of which #2 is usually a minor issue, but it's just funny how spectacularly bad
#pgp is